Strong reference cycles between classes in Swift - Mini Project: Build & Apply

Strong reference cycles between classes

📖 Scenario: Imagine you are building a simple app to manage a library system. You want to keep track of Authors and their Books. Each author can have multiple books, and each book has one author.

🎯 Goal: You will create two classes, Author and Book, that reference each other. Then, you will see how a strong reference cycle can happen and how to fix it using weak references.

📋 What You'll Learn

Create a class called Author with a name property and a books array property.

Create a class called Book with a title property and an author property.

Create an Author instance and a Book instance that reference each other.

Fix the strong reference cycle by making the author property in Book a weak reference.

Print messages when Author and Book instances are deinitialized to observe memory management.

💡 Why This Matters

🌍 Real World

Strong reference cycles can cause memory leaks in apps, making them slower or crash. Understanding how to avoid them is important when building apps that manage related objects.

💼 Career

Swift developers must know how to manage memory and avoid strong reference cycles to write efficient and reliable iOS apps.

1

Create the Author and Book classes

Create a class called Author with a name property of type String and a books property as an empty array of Book. Create a class called Book with a title property of type String and an author property of type Author? (optional).

Swift

# Your code here

Hint

Define two classes with the exact property names and types as described.

2

Create instances and link them

Create an Author instance called author with the name "Jane Austen". Create a Book instance called book with the title "Pride and Prejudice". Set the author property of book to the author instance. Add book to the books array of author.

Swift

class Author {
    let name: String
    var books: [Book] = []
    
    init(name: String) {
        self.name = name
    }
}

class Book {
    let title: String
    var author: Author?
    
    init(title: String) {
        self.title = title
    }
}

// Create author and book instances and link them
# Your code here

Hint

Create the instances with the exact names and values, then link them as described.

3

Add deinitializers to observe memory management

Add deinit methods to both Author and Book classes that print "Author deinitialized" and "Book deinitialized" respectively.

Swift

class Author {
    let name: String
    var books: [Book] = []
    
    init(name: String) {
        self.name = name
    }
    
    // Add deinit here
    # Your code here
}

class Book {
    let title: String
    var author: Author?
    
    init(title: String) {
        self.title = title
    }
    
    // Add deinit here
    # Your code here
}

let author = Author(name: "Jane Austen")
let book = Book(title: "Pride and Prejudice")
book.author = author
author.books.append(book)

Hint

Add deinit blocks inside both classes that print the required messages.

4

Fix the strong reference cycle with a weak reference

Change the author property in the Book class to be a weak optional reference by adding the weak keyword before var author: Author?. Then, set both author and book to nil and print "End of program".

Swift

class Author {
    let name: String
    var books: [Book] = []
    
    init(name: String) {
        self.name = name
    }
    
    deinit {
        print("Author deinitialized")
    }
}

class Book {
    let title: String
    // Change author to weak reference
    # Your code here
    
    init(title: String) {
        self.title = title
    }
    
    deinit {
        print("Book deinitialized")
    }
}

var author: Author? = Author(name: "Jane Austen")
var book: Book? = Book(title: "Pride and Prejudice")
book?.author = author
author?.books.append(book!)

// Set author and book to nil
# Your code here

print("End of program")

Hint

Make the author property weak and then set both variables to nil to see the deinitializers called.

Practice

1.

What causes a strong reference cycle between two Swift classes?

Consider two classes holding references to each other.

easy

A. Neither class holds any reference to the other.

B. One class holds a weak reference and the other a strong reference.

C. Both classes hold strong references to each other, preventing deallocation.

D. Both classes hold weak references to each other.

Solution

1. Step 1: Understand strong references

   Strong references keep objects alive by increasing their reference count.

2. Step 2: Identify cycle condition

   If two objects hold strong references to each other, neither can be deallocated, causing a cycle.

3. Final Answer:

   Both classes hold strong references to each other, preventing deallocation. -> Option C

4. Quick Check:

   Strong references + mutual hold = cycle [OK]

Hint: Strong references both ways cause cycles; use weak/unowned [OK]

Common Mistakes:

• Thinking weak references cause cycles
• Assuming one strong and one weak causes cycle
• Confusing unowned with strong references

2.

Which of the following is the correct way to declare a weak reference in Swift to avoid a strong reference cycle?

class Person {
    var apartment: Apartment?
}

class Apartment {
    // What should go here?
    var tenant: ???
}

easy

A. var tenant: Person

B. weak var tenant: Person?

C. unowned var tenant: Person

D. var tenant: Person?

Solution

1. Step 1: Identify weak reference syntax

   In Swift, weak references are declared with the keyword weak and must be optional.

2. Step 2: Apply to property

   So, weak var tenant: Person? is correct to avoid strong cycles.

3. Final Answer:

   weak var tenant: Person? -> Option B

4. Quick Check:

   Weak + optional = correct syntax [OK]

Hint: Weak references must be optional and use 'weak' keyword [OK]

Common Mistakes:

• Declaring weak without optional type
• Using unowned when weak is needed
• Forgetting 'weak' keyword

3.

What will be the output of the following Swift code?

class Person {
    let name: String
    var apartment: Apartment?
    init(name: String) { self.name = name }
    deinit { print("Person \(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    weak var tenant: Person?
    init(unit: String) { self.unit = unit }
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person? = Person(name: "John")
var apt: Apartment? = Apartment(unit: "4A")
john!.apartment = apt
apt!.tenant = john
john = nil
apt = nil

medium

A. Person John is being deinitialized
Apartment 4A is being deinitialized

B. No output, memory leak occurs

C. Only Person John is being deinitialized

D. Only Apartment 4A is being deinitialized

Solution

1. Step 1: Analyze references

   Person holds strong reference to Apartment; Apartment holds weak reference to Person.

2. Step 2: Check deallocation

   Setting both variables to nil breaks strong references, so both objects deinit and print messages.

3. Final Answer:

   Person John is being deinitialized Apartment 4A is being deinitialized -> Option A

4. Quick Check:

   Weak reference avoids cycle, both deinit [OK]

Hint: Weak reference breaks cycle; both objects deinit when nil [OK]

Common Mistakes:

• Expecting memory leak output
• Thinking only one object deinitializes
• Ignoring weak reference effect

4.

Identify the error in the following Swift code that causes a strong reference cycle:

class Customer {
    let name: String
    var card: CreditCard?
    init(name: String) { self.name = name }
    deinit { print("Customer \(name) is being deinitialized") }
}

class CreditCard {
    let number: UInt64
    var customer: Customer?
    init(number: UInt64) { self.number = number }
    deinit { print("CreditCard \(number) is being deinitialized") }
}

var alice: Customer? = Customer(name: "Alice")
var card: CreditCard? = CreditCard(number: 1234567890123456)
alice!.card = card
card!.customer = alice
alice = nil
card = nil

medium

A. CreditCard's customer property should be declared weak to avoid cycle.

B. Customer's card property should be declared weak to avoid cycle.

C. Both properties should be unowned to avoid cycle.

D. No error; code works fine without changes.

Solution

1. Step 1: Identify reference types

   Both Customer.card and CreditCard.customer are strong references by default.

2. Step 2: Find cycle cause

   Since both hold strong references, they create a cycle preventing deallocation.

3. Step 3: Fix cycle

   Declaring CreditCard.customer as weak var customer: Customer? breaks the cycle.

4. Final Answer:

   CreditCard's customer property should be declared weak to avoid cycle. -> Option A

5. Quick Check:

   Weak reference breaks cycle [OK]

Hint: Make one side weak to break cycle, usually the back reference [OK]

Common Mistakes:

• Making both references weak
• Making the wrong property weak
• Ignoring the cycle causing deallocation failure

5.

You have two classes, Owner and Car. The Owner has a strong reference to Car, and Car has a strong reference to Owner. You want to avoid a strong reference cycle but also ensure that Car always has an Owner during its lifetime. How should you declare the reference from Car to Owner?

hard

A. Declare the reference as weak var owner: Owner (non-optional)

B. Declare the reference as unowned var owner: Owner

C. Declare the reference as var owner: Owner (strong reference)

D. Declare the reference as weak var owner: Owner?

Solution

1. Step 1: Understand ownership requirements

   Since Car must always have an Owner during its lifetime, the reference cannot be optional.

2. Step 2: Choose reference type

   unowned references are non-optional and do not increase reference count, avoiding cycles.

3. Step 3: Apply declaration

   Declare unowned var owner: Owner in Car to avoid cycle and ensure non-optional reference.

4. Final Answer:

   Declare the reference as unowned var owner: Owner -> Option D

5. Quick Check:

   Unowned + non-optional = correct for guaranteed lifetime [OK]

Hint: Use unowned for non-optional back references to avoid cycles [OK]

Common Mistakes:

• Using weak with non-optional type (invalid)
• Using strong reference causing cycle
• Using weak optional when non-optional needed